The present invention concerns a method for regulating the common electric supply of several electric machines each capable of driving a driving wheel of a motor vehicle, wherein a polyphase AC supply current, whose voltage and/or frequency are regulated as a function of at least one set-value signal and of a reference signal, is produced to supply said electric machines together, a set of speed signals each representing the speed of rotation of a respective driving wheel is continually produced and one of the speed signals is selected from said set to deliver the reference signal.
The invention also concerns a driving system for an electrically powered motor vehicle, in particular for implementing the aforementioned method, said system comprising several electric machines each coupled to a driving wheel of the vehicle, a supply device producing a polyphase AC supply current to supply said electric machines together, and a regulating device for controlling the supply device as a function of a set-value drive signal and the rotation speeds of the driving wheels, the regulating device comprising means for continually producing a set of speed signals representing respectively the speeds of each driving wheel, processing means arranged for delivering a reference signal as a function of the speed signals, and a control unit arranged for receiving the set-value traction signal and the reference signal and consequently for controlling the supply device.
Patent application EP-A-0 576 947 discloses such a driving system for a two or four wheel drive electric motor vehicle. This system is illustrated schematically by FIG. 1 annexed hereto for the case of a two wheel drive vehicle. Each driving wheel is coupled to its own polyphase asynchronous motor M1, M2, all of these motors being connected to a central electric supply 2 which produces a polyphase current IAL having a voltage UAL and a frequency FAL, which are variable by pulse width modulation (PWM) under the control of an electronic regulating device. This device receives as inputs, on the one hand, set-value traction signals SPP and set-value braking signals SFN, produced for example by the driver with the aid of a control pedal, and on the other hand, a signal SMI representative of the polyphase supply current and a set of frequency signals MFR1, MFR2 each of which comes from a respective sensor 10 and represents the instantaneous speed of corresponding motor M1, M2 and of the wheel associated therewith. The device processes the input signals to deliver to central supply 2 two control signals SCF and SCA which define respectively the frequency and the amplitude, and thus the effective voltage of the common supply of the motors.
Given that the wheels may rotate at different speeds, in particular when the vehicle turns a corner, one of said frequency signals (i.e. one of the speed signal) MFR1, MFR2 is selected by a processing unit 4 as reference signal SFMX to be used for regulating by a control unit 6, whereas the other signal or signals of the set of signals remain unused. According to the aforecited document, the highest frequency signal will always be selected, so that regulation of all the motors is effected in real time by considering only the fastest rotating motor and wheel.
This known method is advantageous both in drive mode and in electric braking mode, in association with asynchronous motors, because these latter only produce torque if there exists a difference in rotation speed between the rotor and the stator rotating field, such difference being called slip. In drive mode, the fastest rotating motor has the lowest slip and thus provides the lowest engine torque. The corresponding wheel thus has less tendency to skid than the others and, if it does skid anyway because of insufficient grip, the slip of its motor and thus also of the torque decreases, which encourages the wheel to regain grip. Consequently, in most cases of skidding in drive mode, the speed of the wheel which is skidding does not increase much, so that the regulating device does not increase the supply frequency much and the engine torque subsists on the other driving wheel or wheels.
It may however happen that one driving wheel (or several) skid long enough for the regulating device to "race" by repeatedly increasing the common supply frequency in drive mode. In particular if one driving wheel skids on a very slippery surface such as ice, it continues to be driven almost at the speed of the rotating field, with a low torque and low motor slip. Since the regulating device defines the set-value frequency by adding a substantial slip frequency to the reference signal frequency provided by the fastest rotating wheel, it then tends to increase the supply frequency, and thus the speed of the skidding wheel, thus once again the supply frequency and so on. In the other motors, the slip becomes so great that the torque ends up decreasing on the wheel or wheels which are not skidding, so that proper traction is no longer possible. This annoying situation cannot end without external action, for example a change in the set-value signal because the driver significantly releases the accelerator, or without the use of a generally complex and expensive anti-skid device, such as is known for conventional motor vehicles.
In braking mode, the principle of selecting the signal representating the fastest rotating wheel is advantageous in the event that a wheel having insufficient grip is locked. By the term "locked", one means here that the rotation of the wheel is stopped, for example by mechanical braking, or excessively slowed down by electric braking, so that it skids on the ground because its rotation speed rapidly falls to a much lower value than that which would result from the running speed of the vehicle. With the aforementioned principle of selection, the speed signal of a locked wheel will not be selected. However, the device disclosed in the aforecited document cannot operate properly if all the driving wheels are locked, because it then continues to brake the wheels electrically by maintaining negative slip in the motors.
An object of the present invention is to improve such a method and such a device so as to prevent automatically one or more driving wheels racing in the event of skidding in drive mode, by using relatively simple, reliable and inexpensive technical means. According to an additional object of the invention, the method and the device should allow operation according to a similar principle in electric braking mode.
For this purpose, the invention provides a method of the type indicated hereinbefore, characterised in that, in a drive mode where said machines drive the driving wheels, skid detection is continually being effected for each driving wheel and, in the selection step, if at least one driving wheel is skidding and at least one driving wheel is not skidding, the speed signal corresponding to the fastest rotating non-skidding driving wheel is selected or, if all the driving wheels are skidding, a first substitution signal is used in order to deliver the reference signal.
Thus, in the event that, for example a single wheel is skidding in drive mode, the corresponding speed signal is simply left to one side and the selection is made from the other speed signal or signals according to the method of the prior art, with the same advantages and without risk of racing. If all the wheels are skidding, regulating may be continued rationally with the aid of a substitution signal which represents, for example, the rotation speed of a driving wheel immediately prior to skidding.
According to an advantageous development of the method, in a braking mode where said machines are driven by the driving wheels, locking detection is continually effected for each driving wheel and, in the selection step, if at least one driving wheel is not locked, the speed signal corresponding to the fastest rotating driving wheel is selected or, if all the driving wheels are locked, a second substitution signal is used to deliver the reference signal.
As a result, similar selection rules are applied in drive mode and in braking mode, since the case where all the driving wheels are locked is resolved in a similar manner to the case in which all the driving wheels are skidding. The second substitution signal preferably represents the rotation speed of a driving wheel immediately prior to locking, so that it can be determined in the same manner as the first substitution signal.
A driving system according to the invention is characterised in that the processing means comprise selecting means and, for each speed signal, a skid detection unit arranged for detecting skidding via detection of an abrupt rise in the speed signal and for delivering to the selecting means a corrected speed signal and a skid signal, the selecting means being arranged for selecting one of the corrected speed signals as the reference signal as a function of the skid signals.
Each skid detection unit is preferably also arranged for detecting locking of the corresponding wheel via detection of an abrupt decrease in the speed signal and for delivering to the selecting means a locking signal, the selecting means being arranged for selecting one of the corrected speed signals as the reference signal as a function of the skid signals and the locking signals.